Membrane electrode assembly (MEA) is an assembled stack of proton exchange membrane (PEM) or alkali ion exchange membrane (AAEM), a catalyst layer and a gas diffusion layer (GDL) used one over another to form a self-contained module. The PEM is sandwiched between two GDLs which have the catalyst embedded in them. These two electrodes serve as the anode and cathode respectively. The PEM is a proton permeable but electrically insulating barrier. This barrier allows the transport of the protons from the anode to the cathode through the membrane but forces the electrons to travel around a conductive external path to the cathode. In this way, the electrodes are electrically insulated from each other. Many companies are in production of both PEMs as well as fuel cells using PEMs. Nafion is the popular PEM manufactured by DuPont [http://www.electronics.ca/publication/product/directoryand company profiles-%252d Fuel Cells Hydrogen energy and related nanotechnologies. Html].
Platinum is one of the most commonly used catalysts; however other metals like rhodium and ruthenium are also used. Since the high costs of these and other similar materials are still a hindering factor in the wide spread economical acceptance of fuel cell technology, research is being undertaken to develop catalysts that use lower cost materials. The conventional process involved in the preparation of MEA comprises hot pressing of the electrodes onto the PEM [Kim et al. US Patent 20100279196, Swathirajan et al. US Patent 531687, Popov et al. US Patent 2006/0040157 A1]. Commonly used materials for the GDL are carbon coated carbon cloth or Toray carbon fiber paper. The conventional ways of direct transfer of catalyst layers on the proton conducting membranes containing doped phosphoric acid has practical limitations due to the surface wetness caused by phosphoric acid segregation on the membrane surface [L. Qingfeng et al., J. Appl. Electrochem. 31 (2001) 773-779, O. E. Kongstein et al, Energy 32 (2007) 418-422]. On the other hand, the brush coating method of catalyst layer transfer requires unnecessarily high platinum loading (1-2 mg/cm2) to maintain reasonable performance characteristics.
Therefore, the present inventors have come up with a process of direct-transfer of the ‘catalyst layer’ onto the phosphoric acid doped polybenzimidazole (PBI) membrane by surmounting the limitations possessed by such systems. By following the present invention, the platinum loading can be reduced as low as 0.5 mg/cm2, while retaining high performance characteristics like current density and power density at a given temperature. The present process will be useful to other membranes as well, if the surface wetness causes any practical limitation to effectively generate the catalyst layer by direct transfer.
L. Qingfeng et al., J. Appl. Electrochem. 31 (2001) 773-779.
O. E. Kongstein et al, Energy 32 (2007) 418-422
U.S. Pat. No. 6,946,211 in Example 2 discloses that on to the supporting layer of the carbon paper by tape-casting was applied a mixture of 40 wt % Pt/C catalyst powder and 60 wt % FBI from a 3 wt % polymer solution in dimethylacetamide. The platinum loading in the catalyst layer is 0.45 mg/cm.sup.2. After drying at 130 .degree. C. for 10 minutes, the electrode was impregnated with a mixed acid of 65 wt % phosphoric acid and 35 wt % trifluoroacetic acid. The amount of impregnated phosphoric acid is related to the FBI content in the catalyst layer of the electrode, in a molar ratio of 14 to 1. From the impregnated electrodes and acid-doped PBI membranes (doping level 650), a membrane-electrode assembly was made by means of hot-press at a temperature of 150 .degree. C., a pressure of 0.5 bar, and a duration of 12 minutes.
The heating of the phosphoric acid doped PBI membrane at 160° C. to remove the excess phosphoric acid thereby achieving 100% catalyst transfer onto FBI from non-porous support is a technique undisclosed hitherto in prior arts.